| Year | Value | | 2021 | 9.406 | | 2020 | 9.33 | | 2019 | 6.931 | | 2018 | 6.856 | | 2017 | 6.615 | | 2016 | 6.305 | | 2015 | 6.007 | | 2014 | 5.805 | | 2013 | 5.696 | | 2012 | 5.642 | | 2011 | 5.597 | | 2010 | 5.594 | | 2009 | 5.533 | | 2008 | 5.426 | | 2007 | 5.302 | | 2006 | 5.177 | | 2005 | 5.078 | | 2004 | 5.001 | | 2003 | 4.96 | | 2002 | 4.946 | | 2001 | 4.962 | | 2000 | 4.997 | | 1999 | 5.036 | | 1998 | 5.086 | | 1997 | 5.152 | | 1996 | 5.235 | | 1995 | 5.316 | | 1994 | 5.403 | | 1993 | 5.489 | | 1992 | 5.584 | | 1991 | 5.68 | | 1990 | 5.777 | | 1989 | 5.855 | | 1988 | 5.938 | | 1987 | 6.051 | | 1986 | 6.186 | | 1985 | 6.341 | | 1984 | 6.534 | | 1983 | 6.772 | | 1982 | 7.04 | | 1981 | 7.32 | | 1980 | 7.573 | | 1979 | 7.786 | | 1978 | 7.975 | | 1977 | 8.125 | | 1976 | 8.274 | | 1975 | 8.432 | | 1974 | 8.625 | | 1973 | 8.841 | | 1972 | 9.075 | | 1971 | 9.305 | | 1970 | 9.605 | | 1969 | 9.895 | | 1968 | 10.172 | | 1967 | 10.454 | | 1966 | 10.759 | | 1965 | 11.084 | | 1964 | 11.447 | | 1963 | 11.856 | | 1962 | 12.323 | | 1961 | 12.844 | | 1960 | 13.435 |
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